1,316 research outputs found
Extending Hybrid CSP with Probability and Stochasticity
Probabilistic and stochastic behavior are omnipresent in computer controlled
systems, in particular, so-called safety-critical hybrid systems, because of
fundamental properties of nature, uncertain environments, or simplifications to
overcome complexity. Tightly intertwining discrete, continuous and stochastic
dynamics complicates modelling, analysis and verification of stochastic hybrid
systems (SHSs). In the literature, this issue has been extensively
investigated, but unfortunately it still remains challenging as no promising
general solutions are available yet. In this paper, we give our effort by
proposing a general compositional approach for modelling and verification of
SHSs. First, we extend Hybrid CSP (HCSP), a very expressive and process
algebra-like formal modeling language for hybrid systems, by introducing
probability and stochasticity to model SHSs, which is called stochastic HCSP
(SHCSP). To this end, ordinary differential equations (ODEs) are generalized by
stochastic differential equations (SDEs) and non-deterministic choice is
replaced by probabilistic choice. Then, we extend Hybrid Hoare Logic (HHL) to
specify and reason about SHCSP processes. We demonstrate our approach by an
example from real-world.Comment: The conference version of this paper is accepted by SETTA 201
Uniform Labeled Transition Systems for Nondeterministic, Probabilistic, and Stochastic Process Calculi
Labeled transition systems are typically used to represent the behavior of
nondeterministic processes, with labeled transitions defining a one-step state
to-state reachability relation. This model has been recently made more general
by modifying the transition relation in such a way that it associates with any
source state and transition label a reachability distribution, i.e., a function
mapping each possible target state to a value of some domain that expresses the
degree of one-step reachability of that target state. In this extended
abstract, we show how the resulting model, called ULTraS from Uniform Labeled
Transition System, can be naturally used to give semantics to a fully
nondeterministic, a fully probabilistic, and a fully stochastic variant of a
CSP-like process language.Comment: In Proceedings PACO 2011, arXiv:1108.145
Process Algebras
Process Algebras are mathematically rigorous languages with well defined semantics that permit describing and verifying properties of concurrent communicating systems.
They can be seen as models of processes, regarded as agents that act and interact continuously with other similar agents and with their common environment. The agents may be real-world objects (even people), or they may be artifacts, embodied perhaps in computer hardware or software systems.
Many different approaches (operational, denotational, algebraic) are taken for describing the meaning of processes. However, the operational approach is the reference one. By relying on the so called Structural Operational Semantics (SOS), labelled transition systems are built and composed by using the different operators of the many different process algebras. Behavioral equivalences are used to abstract from unwanted details and identify those systems that react similarly to external
experiments
A Denotational Semantics for Communicating Unstructured Code
An important property of programming language semantics is that they should
be compositional. However, unstructured low-level code contains goto-like
commands making it hard to define a semantics that is compositional. In this
paper, we follow the ideas of Saabas and Uustalu to structure low-level code.
This gives us the possibility to define a compositional denotational semantics
based on least fixed points to allow for the use of inductive verification
methods. We capture the semantics of communication using finite traces similar
to the denotations of CSP. In addition, we examine properties of this semantics
and give an example that demonstrates reasoning about communication and jumps.
With this semantics, we lay the foundations for a proof calculus that captures
both, the semantics of unstructured low-level code and communication.Comment: In Proceedings FESCA 2015, arXiv:1503.0437
Process algebra for performance evaluation
This paper surveys the theoretical developments in the field of stochastic process algebras, process algebras where action occurrences may be subject to a delay that is determined by a random variable. A huge class of resource-sharing systems – like large-scale computers, client–server architectures, networks – can accurately be described using such stochastic specification formalisms. The main emphasis of this paper is the treatment of operational semantics, notions of equivalence, and (sound and complete) axiomatisations of these equivalences for different types of Markovian process algebras, where delays are governed by exponential distributions. Starting from a simple actionless algebra for describing time-homogeneous continuous-time Markov chains, we consider the integration of actions and random delays both as a single entity (like in known Markovian process algebras like TIPP, PEPA and EMPA) and as separate entities (like in the timed process algebras timed CSP and TCCS). In total we consider four related calculi and investigate their relationship to existing Markovian process algebras. We also briefly indicate how one can profit from the separation of time and actions when incorporating more general, non-Markovian distributions
Using Indexed and Synchronous Events to Model and Validate Cyber-Physical Systems
Timed Transition Models (TTMs) are event-based descriptions for modelling,
specifying, and verifying discrete real-time systems. An event can be
spontaneous, fair, or timed with specified bounds. TTMs have a textual syntax,
an operational semantics, and an automated tool supporting linear-time temporal
logic. We extend TTMs and its tool with two novel modelling features for
writing high-level specifications: indexed events and synchronous events.
Indexed events allow for concise description of behaviour common to a set of
actors. The indexing construct allows us to select a specific actor and to
specify a temporal property for that actor. We use indexed events to validate
the requirements of a train control system. Synchronous events allow developers
to decompose simultaneous state updates into actions of separate events. To
specify the intended data flow among synchronized actions, we use primed
variables to reference the post-state (i.e., one resulted from taking the
synchronized actions). The TTM tool automatically infers the data flow from
synchronous events, and reports errors on inconsistencies due to circular data
flow. We use synchronous events to validate part of the requirements of a
nuclear shutdown system. In both case studies, we show how the new notation
facilitates the formal validation of system requirements, and use the TTM tool
to verify safety, liveness, and real-time properties.Comment: In Proceedings ESSS 2015, arXiv:1506.0325
On the Expressiveness of Markovian Process Calculi with Durational and Durationless Actions
Several Markovian process calculi have been proposed in the literature, which
differ from each other for various aspects. With regard to the action
representation, we distinguish between integrated-time Markovian process
calculi, in which every action has an exponentially distributed duration
associated with it, and orthogonal-time Markovian process calculi, in which
action execution is separated from time passing. Similar to deterministically
timed process calculi, we show that these two options are not irreconcilable by
exhibiting three mappings from an integrated-time Markovian process calculus to
an orthogonal-time Markovian process calculus that preserve the behavioral
equivalence of process terms under different interpretations of action
execution: eagerness, laziness, and maximal progress. The mappings are limited
to classes of process terms of the integrated-time Markovian process calculus
with restrictions on parallel composition and do not involve the full
capability of the orthogonal-time Markovian process calculus of expressing
nondeterministic choices, thus elucidating the only two important differences
between the two calculi: their synchronization disciplines and their ways of
solving choices
Machine-Checkable Timed CSP
The correctness of safety-critical embedded software is crucial, whereas non-functional properties like deadlock-freedom and real-time constraints are particularly important. The real-time calculus Timed Communicating Sequential Processes (CSP) is capable of expressing such properties and can therefore be used to verify embedded software. In this paper, we present our formalization of Timed CSP in the Isabelle/HOL theorem prover, which we have formulated as an operational coalgebraic semantics together with bisimulation equivalences and coalgebraic invariants. Furthermore, we apply these techniques in an abstract specification with real-time constraints, which is the basis for current work in which we verify the components of a simple real-time operating system deployed on a satellite
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